Field of the Invention
The present invention relates generally to computers, and more particularly, to fabricating key fields in a computing storage environment.
Description of the Related Art
In today's society, computer systems are commonplace. Computer systems may be found in the workplace, at home, or at school. Computer systems may include data storage systems, or disk storage systems, to process and store data. Data storage systems, or disk storage systems, are utilized to process and store data. A storage system may include one or more disk drives. The disk drives may be configured in an array, such as a Redundant Array of Independent Disks (RAID) topology, to provide data security in the event of a hardware or software failure. The data storage systems may be connected to a host, such as a mainframe computer. The disk drives in many data storage systems have commonly been known as Direct Access Storage Devices (DASD). DASD devices typically store data on a track, which is a circular path on the surface of a disk on which information is recorded and from which recorded information is read.
These disk drives implement a Count, Key, and Data (CKD) format on the disk drives. A record is a set of one or more related data items grouped together for processing, such that the group may be treated as a unit. Disk drives utilizing the CKD format have a special “address mark” on each track that signifies the beginning of a record on the track. After the address mark is a three-part record beginning with the count field that serves as the record ID and also indicates the lengths of the optional key field and the data field, both of which follow. Also on the track, there is normally one Home Address (HA) that defines the physical location of the track and the condition of the track. The HA typically contains the physical track address, a track condition flag, a cylinder number (CC) and a head number (HH). The combination of the cylinder number and the head number indicates the track address, commonly expressed in the form CCHH. The HA contains the “physical track address” which is distinguished from a “logical track address.” Some operating systems, such as the IBM Virtual Machine (VM) operating system, employ a concept of “virtual disks” referred to as user mini-disks, and thus it is necessary to employ logical addresses for the cylinders rather than physical addresses.
In addition, within a storage area network, Write Ahead Data Set (WADS) tracks may be included. The write-ahead data set (WADS) is a small DASD data set containing a copy of log records reflecting committed operations in the on-line data set OLDS buffers that have not yet been written to the OLDS. WADS space is continually reused after the records it contains are written to the OLDS.
The computer systems, as previously described, use Input/output (I/O) operations to transfer data between memory and input/output devices of a processing environment. Specifically, data is written from memory to one or more input/output devices, and data is read from one or more input/output devices to memory by executing input/output operations. To facilitate processing of input/output operations, an input/output subsystem of the processing environment is employed. The input/output subsystem is coupled to main memory and the input/output devices of the processing environment and directs the flow of information between memory and the input/output devices. One example of an input/output subsystem is a channel subsystem. The channel subsystem uses channel paths as communications media. Each channel path includes a channel coupled to a control unit; the control unit being further coupled to one or more input/output devices.
The channel subsystem employs channel command words to transfer data between the input/output devices and memory. A channel command word (CCW) specifies the command to be executed, and for commands initiating certain I/O operations, it designates the memory area associated with the operation, the action to be taken whenever transfer to or from the area is completed, and other options.
During input/output processing, a list of channel command words is fetched from memory by a channel. The channel parses each command from the list of channel command words and forwards a number of the commands, each command in it's own entity, to a control unit (processor) coupled to the channel. The control unit then processes the commands. The channel tracks the state of each command and controls when the next set of commands are to be sent to the control unit for processing. The channel ensures that each command is sent to the control unit in it's own entity. Further, the channel infers certain information associated with processing.